Apparatus for the preparation of a foamed article of a polyolefin resin

ABSTRACT

A sheet of a cross-linkable and foamable polyolefin resin is thermally treated on an endless conveyor belt formed of a plain weave wire net. A plurality of parallel, spaced apart rolls are disposed in a direction perpendicular to the direction in which the conveyor belt runs for supporting engagement and rolling contact with the conveyor belt to maintain the flatness of the conveyor belt.

This invention relates generally to an apparatus and a method for thepreparation of a foamed article of a polyolefin resin. Moreparticularly, the present invention is concerned with an apparatus and amethod for thermally treating a sheet of a cross-linkable and foamablepolyolefin resin to effect cross-linking thereof.

Foamed articles of a polyolefin resin are generally prepared by a methodincluding the steps of: providing a mixture containing a polyolefin, across-linking agent such as an organic peroxide and a blowing agent;forming the mixture into a desired shape such as into a sheet; placingthe sheet on a flat support member such as a belt conveyor; heattreating the sheet provided on the support member at a first temperatureby, for example, contact with a hot gas, to effect cross-linking andthen at a second temperature to decompose the blowing agent and to allowthe sheet to foam and expand. As the support member, both gas permeableand impermeable materials have been generally used. Although agas-impermeable plate such as a metal plate has an excellent flatness,there is a considerable difference in temperature between the backsideof the sheet in contact with the metal plate and the front side directlyexposed to the hot gas. As a consequence, there is a difference betweenthe backside and the front side in rates at which the cross-linking andthe succeeding foaming proceed, resulting in the lack of uniformity ofcell structures. In contrast, with a gas permeable belt such as a wirenet, the sheet placed thereon can be heated substantially evenly fromboth sides of the sheet so that the sheet can cross-link and foamuniformly.

However, the use of an endless belt formed of a wire net has now beenfound to encounter the following problems:

1. Removal of the cross-linked sheet from the wire net involves adifficulty because of the intrusion of the sheet into the openings ofthe net during the heat treatment.

2. The mark or impression of the wire net pattern is formed on thesurface of the cross-linked sheet, the mark becoming more obvious afterfoaming.

3. The joint portion of the endless belt causes uneveness of heating.

4. The wire net belt tends to slack during use, causing deformation ofthe sheet.

With regard to the problem 4 above, the loosening of the belt may beprevented by lining it with a reinforcement such as a rigid wire nethaving a coarse mesh and formed of thick wires. However, such areinforcement decreases the area of the openings of the belt throughwhich the hot gas flows for heating the sheet provided thereon, causinga difference in temperature between the front and back sides of thesheet. Moreover, such a reinforcement brings about variation intemperature and in cross-linking rate on the backside of the sheetbecause the position of the sheet relative to the reinforcement is keptunchanged throughout the passage of the sheet on the conveyor belt.

The present invention has been made with a consideration of the aboveproblems of the conventional technique.

In accordance with the present invention there is provided an apparatusfor thermally treating a sheet of a cross-linkable and foamablepolyolefin resin, comprising:

a furnace;

a pair of parallel endless roller chains having an endless conveyor beltwhich is formed of a plain weave wire net and which is disposed betweenand connected to said pair of endless roller chains for movementtherewith, each of said endless roller chains being in mesh with a drivesprocket and one or more idler sprockets spaced therefrom so that thereis defined within said furnace a substantially flat path between saidpair of endless roller chains along which said conveyor belt can runwhile supporting the sheet thereon;

a plurality of parallel, spaced apart rolls each oriented in a directionperpendicular to the direction in which said conveyor belt runs alongsaid path and disposed adjacent to said path for supporting engagementand rolling contact with said conveyor belt running along said path; and

first heating means provided in said furnace for heating the sheet fedonto said conveyor belt running along said path so that the sheet onsaid conveyor belt may undergo cross-linking during its passage throughsaid path.

In another aspect, the present invention provides a method ofcross-linking a sheet of cross-linkable and foamable polyolefin resin,comprising:

introducing the sheet into a furnace comprising an endless conveyor belttravelling along a predetermined path including a substantially flatpath and formed of a plain weave wire net having a mesh size of between20 and 70 mesh and a substantially uniform thickness;

feeding the sheet onto said endless belt travelling along said flat pathto allow said sheet to travel therewith; and

heating said sheet travelling along said flat path at a temperature sothat said sheet may undergo cross-linking.

The present invention will be described in detail below with referenceto the accompanying drawings in which:

FIG. 1 is a vertical, cross-sectional view diagrammatically showing oneembodiment of thermal treatment apparatus according to the presentinvention;

FIG. 2 is a perspective view schematically showing the belt conveyor ofthe apparatus shown in FIG. 1;

FIGS. 3(a) through 3(g) are plan views showing various weaves of wires,FIG. 3(a) being plain weave suitable for the present invention;

FIG. 4 is a side view schematically showing the joint portion of aconventional endless wire belt;

FIG. 5 is a side view, similar to FIG. 4, showing the joint portion ofthe endless conveyor belt according to the present invention;

FIG. 6 is a fragmentary, cross-sectional view taken along line VI--VI inFIG. 2;

FIG. 7 is a vertical cross-sectional view, similar to FIG. 1, showinganother embodiment according to the present invention; and

FIGS. 8 and 9 are vertical cross-sectional views, similar to FIG. 7,showing further embodiments of the present invention.

Referring now to FIG. 1, the reference numeral 1 denotes a furnacewithin which a sheet 2 of a cross-linkable, foamable polyolefin resinintroduced thereinto through an inlet 3 by any suitable means, such as apair of feed rolls 20, is thermally treated for effecting cross-linkingand, if desired, foaming of the sheet. The heat-treated sheet isdischarged through an outlet 4 from the furnace 1 with the aid of anysuitable means such as a pair of discharge rolls 21.

Disposed within the furnace 1 is an endless conveyor belt 5 forsupporting the sheet 2 thereon and for conveying same through thefurnace 1. Details of the conveyor belt arrangement is shown in FIG. 2,in which designated as 6c, 7c, 8c and 9c are four parallel shafts whichare journalled on a base frame (not shown) and to which are fixedlysecured pairs of sprockets 6a and 6b, 7a and 7b, 8a and 8b and 9a and9b, respectively. The shaft 6c is operatively connected to a drive shaftof a drive motor (not shown) so that the sprockets 6a and 6b function asdrive sprockets while the other sprockets serve as idler sprockets. Apair of endless roller chains 10a and 10b are in mesh with the sprockets6a, 7a, 8a and 9a and 6b, 7b, 8b and 9b, respectively, and arepositioned in parallel with each other.

The endless conveyor belt 5 is wound around four rollers 6d, 7d, 8d and9d coaxially supported on the shafts 6c, 7c, 8c and 9c, respectively,and disposed between the pair of endless roller chains 10a and 10b withits both side edges being connected by a plurality of links 11 to theopposite chains 10a and 10b. As a consequence of the above structure,when the pair of roller chains 10a and 10b are moved in the directionshown by the arrow A upon driving the drive sprockets 6a and 6b, theconveyor belt 5 connected thereto moves simultaneously in the samedirection along a predetermined path of travel. The sprockets 6a-9a and6b-9b are so arranged that there is formed a straight, flat path alongwhich the conveyor belt 5 can run while supporting the sheet thereon.

The endless conveyor belt 5 is preferably formed of an elongated, plainweave wire net having such a weave as shown in FIG. 3(a). Both ends ofthe wire net are abutted against and bonded with each other as shown inFIG. 5 to provide the endless structure. The twilled weave net as shownin FIG. 3(b) is inferior to the plain weave net with respect to thereleasability of the cross-linked sheet from the net. Plain Dutch weave(FIG. 3(c)), twilled Dutch weave (FIG. 3(d)) and stranded twill weave(FIG. 3(e)) are ill-suited for the purpose of the present inventionbecause of their low voidage which in turn results in both theunevenness of heating of the sheet placed thereon and the adhesion ofthe sheet thereto. Balance weave (FIG. 3(f)) and Herringbone weave (FIG.3(g)) are also ill-suited because of their roughness of the surfaceswhich causes intrusion of the sheet into the wire net, resulting in thereduction of releasability of the sheet from the net and the formationof impression of the net on the surfaces of the product.

It is preferred that the mesh size of the plain weave wire net bebetween 20 and 70 mesh (between 0.84 and 0.21 mm), more preferablybetween 30 and 60 mesh (between 0.59 and 0.25 mm). With a wire nethaving a mesh size of coarser than 20 mesh, the sheet tends to intrudein the openings of the net. A mesh size of the net of finer than 70 meshtends to cause the adhesion of the sheet to the net because of theincrease in the contact area between them.

As described previously, the endless belt 5 is formed by abutting bothends of an elongated wire net against each other as shown in FIG. 5.Such an abutting engagement is preferable as compared with anoverlapping structure as shown in FIG. 4, because, in the latterstructure, the surface becomes irregular and the thickness becomeslarger at the joining portion. The abutting joint may be effected by anysuitable way such as soldering or sewing with a very fine wire.

Referring again to FIG. 2, a plurality of spaced apart parallel rolls 12are rotatably supported to a base frame 15 (FIG. 6) and arrangedadjacent to the straight path for supporting engagement and rollingcontact with the conveyor belt 5. The rolls 12 serve to improve theflatness of the path along which the conveyor belt 5 travels whilesupporting the sheet to be treated thereon. Moreover, unlike areinforcement provided over the back surface of a wire net for movementtherewith, the rolls 12 do not adversely affect the uniformity inheating the sheet placed on the conveyor belt 5. The distance betweentwo adjacent net support rolls 12 is preferably 100-500 mm.

As shown in FIG. 6, in order to prevent lateral winding of the path oftravel of the conveyor belt 5, the pair of endless roller chains 10a and10b are preferably guided by a pair of parallel guide members 14a and14b extending along the opposite sides of the path, respectively, andmounted on C-shaped portions of the base frame 15. More specifically,each link 11 has a guided neck portion 16 which is inserted through anelongated aperture formed in the guide member 14a (or 14b). As a result,both the vertical and lateral movements of the pair of roller chains 10aand 10b are restricted and the flatness and straightness of the pathalong which the conveyor belt runs are maintained.

Referring again to FIG. 1, heating means 17a and 17b are provided aboveand beneath the straight path of the belt 5. In the illustratedembodiment, the heating means 17a and 17b include ducts 29a and 29b,respectively, having a plurality of transversely extending slits 28a and28b, respectively, from which a hot gas is injected towards the path toheat the sheet 2 placed on the belt 5 so that the sheet 5 can undergocross-linking during its passage through the path. By increasing thelength of the straight path and the heating zone while controlling theretention time and temperature, it is possible to effect foaming of thecross-linked sheet within the same furnace, as illustrated in FIG. 1.The foamed product 19 is discharged from the furnace 1 through theoutlet 4 for recovery.

FIG. 7 shows another embodiment of the present invention in which likereference numerals indicate the same component parts. The apparatusillustrated in FIG. 7 has first heating means 17a and 17b each comprisedof infrared heaters and defining therebetween a first heating zonethrough which the conveyor belt 5 travels while supporting the sheet 2thereon. Disposed adjacent to and downstream of the first heating means17a and 17b are second heating means 27a and 27b including ducts 23a and23b and slits or nozzles 18a and 18b, respectively, from which a hot hassuch as hot air is blown and defining a second heating zonetherebetween. A plurality of parallel support rolls 24 are arranged inthe second heating zone for supporting thereon the sheet supplied fromthe first heating zone. The first heating zone is arranged to heat thesheet to a first temperature sufficient to cross-link the sheet, whilein the second heating zone the cross-linked sheet is heated to a secondtemperature higher than the first temperature so that the sheet may foamand expand.

In heat-treating the cross-linked sheet in the second heating zone, itis preferred that the hot gas injected from the nozzles 18b of the lowerheating means 27b have a greater wind pressure than that from thenozzles 18a of the upper heating means 27a so that the cross-linkedsheet travelling on the rolls 24 may float thereon. Because the sheetexpands upon foaming in three-dimensional directions, the rate at whichthe foamed sheet 19 is discharged from the furnace must be controlled toan appropriate rate greater than that at which the cross-linkable,foamable sheet 2 is fed to the furnace. If the take-up rate is lowerthan the appropriate rate, the sheet in the furnace will wave to causethe formation of wrinkles in the product. On the other hand, a take-upspeed higher than the appropriate rate causes stretching of the sheet,resulting in the reduction in size of the foamed product. However, sincethere may be a fluctuation in the expansion rate of the sheet, it isvery difficult, in practice, to control the take-up rate. The floatingof the sheet on the rolls 24 as mentioned above can compensate such afluctuation to a certain degree and is preferable for reason of easycontrol of the take-up speed.

FIG. 8 illustrates an alternate embodiment, in which like referencenumerals designate like component parts. The furnace 1 of thisembodiment is constituted from a slanted portion in which the sheet 2 issubjected to a cross-linking condition by means of first heating means17a and 17b and a vertical portion in which the cross-linked sheet issubjected to a foaming condition by means of second heating means 27aand 27b. The cross-linked sheet is released from the conveyor belt 5 atthe end of the straight path, i.e. at the roller 7d (FIG. 2) coaxiallyaligned with the rotational axis of the idler sprocket 7a. Thecross-linked sheet is then displaced vertically downward and is passedbetween the second heating means 27a and 27b and undergoes foaming. Thefoamed product is continuously discharged from the furnace 1 by means ofthe discharge rolls 21. The second heating means 27a and 27b in thisembodiment include air ducts 23a and 23b, respectively, havingperforated or porous plates 25a and 25b, respectively, from which a hotgas is blown on the both-surfaces of the vertically running,cross-linked sheet to cause expansion thereof. The reference numerals31a and 31b designate baffles mounted on the perforated plates 25a and25b, respectively, for controlling the direction of the hot gas flow.Designated as 30a and 30b are auxiliary heaters optionally providedbetween the first and second heating means and as 26 a plurality ofguide rolls for preventing the swelling wave (perpendicular movement) ofthe sheet. By streaming a cooled fluid within the guide rolls 26, thefoamed sheet 19 can be cooled during its passage through the guide rolls26. The path along which the conveyor belt 5 runs while supporting thesheet 2 thereon may be upgrade (as shown in FIG. 8), horizontal (asshown in FIG. 9) or downgrade (not shown) and preferably has aninclination of between -30° and +30°.

According to the embodiment shown in FIG. 8, both sides of the sheet canbe heated to substantially the same temperature because of the absenceof any intervening materials, such as rolls 24 of the embodiment in FIG.7, between the heat source and the sheet. Further, without specificcontrol of the wind pressure of the hot gas from the perforated plates25a and 25b, the take-up of the foamed sheet 19 can be performed easilyand smoothly.

FIG. 9 depicts an alternate embodiment of the apparatus shown in FIG. 8,the only difference therebetween being that the sheet 2 to be treated ismaintained in substantially horizontal position in the first heatingzone of the apparatus of FIG. 9. In heat-treating the cross-linkable,foamable sheet in the apparatuses of FIGS. 8 and 9 in which thecross-linked sheet is maintained in vertical position during the foamingand expanding step, it is preferred that the cross-linking be conductedto provide a cross-linking degree (in terms of gel fraction) at a stagejust after the departure of the sheet from the conveyor belt of at least30%, more preferably 40-70%.

Various kinds of polyolefin resins may be used for the purpose of thepresent invention. Examples of such polyolefin resins includehomopolymers and copolymers of olefins such as ethylene, propylene andbutene; copolymers and terpolymers of olefins and other polymerizablemonomers such as vinyl acetate and acrylic acid; and mixtures of theabove polymers with other polymers such as styrene-butadiene rubbers,acrylonitrile-butadiene-styrene resins, chlorinated polyethylenes andchlorosulfonated polyethylenes. It is preferred that the polyolefinresins have a melt index (MC) of not greater than 2.0 g per 10 min, morepreferably not greater than 1.5 g per 10 min for reasons of freeness ofintrusion of the resin into the openings of the wire net.

The polyolefin resin is mixed with a conventional blowing agent and aconventional cross-linking agent in amounts suitable for providing across-linkable and foamable polyolefin resin. Illustrative of suitablecross-linking agents are organic peroxides such as dicumyl peroxide,2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and tert-butylhydroperoxide;azides; vinyl-group-containing silane compounds; and silanolcondensation catalysts. Blowing agents may be compounds of a type whichis decomposable at an elevated temperature to generate a gas andinclude, for example, p,p'-oxybisbenzenesulfonylhydrazide,azodicarbonamide, N,N'-dinitrosopentamethylenetetramine andazobisisobutyronitrile. The cross-linkable, foamable polyolefin resinmay further contain various other additives such as a foaming assistant,a cross-linking assistant, an antioxidant, a pigment, an antistaticagent and a flame retardant.

In order to improve the releasability of the heat-treated sheet from thewire net conveyor belt, it is advantageous to apply a coatingcomposition containing a releasing agent over the surface of the sheetto be treated which contacts with the belt in an amount of 0.02-1,preferably 0.05-0.5 g per one square meter of the sheet in terms of adry weight. Suitable releasing agents are silicone resins such assilicone oil and fluorine resins such as polytetrafluoroethylene,polychlorotrifluoroethylene, tetrafluoroethylene-hexafluoropropylenecopolymers and polyfluoroinated vinylidenes.

The coating composition may be in the form of a solution, emulsion,suspension, dispersion, paste or the like in a suitable medium such aswater, an organic solvent or a mixture thereof. Above all, the use of anaqueous dispersion or emulsion is preferred for reasons of economy andeasiness in handling. The content of the releasing agent in the coatingcomposition is preferably 0.2-10 wt %, more preferably 0.5-5 wt %. Thecoating composition may contain an additive, especially a surfactantsuch as a fatty acid ester of sucrose. The coating composition can beapplied to the sheet to be treated by any known coating method such asbrush coating, spray coating, roll coating, wire bar coating and dipcoating.

The following examples will further illustrate the present invention.

EXAMPLE 1

A mixture containing 100 parts by weight of a low density polyethylenehaving a melt index of 1.4 g per 10 min, 17 parts by weight ofazodicarbonamide and 0.8 parts by weight of dicumyl peroxide wasextruded to form a cross-linkable and foamable sheet having a width of420 mm and a thickness of 1.8 mm. The sheet was heat-treated in anapparatus as shown in FIG. 8. The conveyor belt was formed of a plainweave wire net having a mesh size of 45 mesh (0.35 mm) and abutted atits opposite ends to form an endless structure. A plurality of rollswere arranged at an interval of about 400 mm for supporting the conveyorbelt and for maintaining a flat, straight path. The running speed of thebelt was 1 m/min. The cross-linking was performed on the conveyor belttravelling along the path with hot air having a temperature of about180° C. The cross-linked sheet was removed from the belt conveyor andwas further heat-treated with hot air having a maximum temperature of245° C. to obtain a foamed sheet having a thickness of 5.0 mm, a widthof 1200 mm and a density of 0.034 g/cm³. The cross-linked sheet was ableto be easily separated from the conveyor belt and smoothly fed to thefoaming zone. Both the upper and lower surfaces of the foamed sheet werevery flat and the lower surface had no impression of the wire netpattern.

EXAMPLE 2

Example 1 was repeated in the same manner as described except thatvarious conveyor belts with different mesh sizes or different weave wereused in lieu of the 45 mesh plain weave wire net. The releasability ofthe cross-linked sheet from the belt, formation of the wire net marks onthe foamed sheet and the flatness of the surface of the foamed sheetwere examined, the results of which are summarized in Table below.

    ______________________________________                                        Ex-                                                                           periment                                                                             Weave    Mesh     Releasea-                                                                            Formation of                                                                            Flat-                               No.    pattern  size     bility wire net mark                                                                           ness                                ______________________________________                                        1      Plain    18       poor   remarkable                                                                              bad                                 2      Plain    45       good   none      good                                3      Plain    75       poor   none      bad                                 4      Twilled  50       poor   Slight    bad                                 5      Plain    10 × 60                                                                          very   none      bad                                        Dutch             bad                                                  6      Twilled  10 × 200                                                                         very   none      very                                       Dutch             bad              bad                                 7      Stranded 30 × 40                                                                          very   none      very                                       Twill             bad              bad                                 8      Balance  --       poor   remarkable                                                                              very                                                                          bad                                 9      Herring- --       Poor   remarkable                                                                              very                                       bone                               bad                                 ______________________________________                                    

EXAMPLE 3

Example 1 was repeated in the same manner as described except that theextrudate sheet was, before being fed to the heat treating apparatus,dipped in an aqueous releasing composition prepared as follows:

Silicone resin coating composition: A commercially available siliconeemulsion (TORAY SILICONE SH7024) was diluted with water to a siliconecontent of 2 wt %, with which was mixed a fatty acid ester of sucrose.

Fluorine resin coating composition: A commercially available aqueouspolytetrafluoroethylene dispersion (TEFLON 30J) was diluted with waterto a polytetrafluoroethylene content of 3 wt %, with which was mixed afatty acid ester of sucrose.

The releasability of the cross-linked sheet from the endless conveyorbelt was found to be improved in both cases.

We claim:
 1. An apparatus for thermally treating a sheet of a cross-linkable and foamable polyolefin resin, comprising:a furnace; a conveyor assembly providing a substantially flat path along which the sheet is supported and conveyed through the furnace, said assembly including:a pair of parallel-disposed endless roller chains having an endless conveyor belt formed of a plain weave wire net disposed between and connected to said chains by a plurality of rigid links for movement therewith, each of said chains being in mesh with a drive sprocket and one or more idler sprockets spaced therefrom; a plurality of parallel, spaced-apart rolls, each stationarily disposed and oriented perpendicularly to the direction of conveyor belt movement and positioned for supporting engagement and rolling contact with said conveyor belt; an elongated guide means provided adjacent to each side of said conveyor belt and engageable by said links and cooperating therewith for preventing both vertical and lateral movement of the links and conveyor belt; and a heating means provided in said furnace for heating the sheet moved on said conveyor belt so that said sheet may undergo cross-linking during its movement through the furnace by said conveyor assembly.
 2. An apparatus as claimed in claim 1, wherein each of said guide means is provided with an elongated slot extending along the length thereof and each of said links having a portion engaged within said slot and coacting therewith to prevent vertical and lateral movement of said link and conveyor belt.
 3. An apparatus as claimed in claim 1, wherein said wire net has a mesh size of between 20 and 70 mesh.
 4. An apparatus as claimed in claim 1, wherein said endless conveyor belt is formed from an elongated sheet of the wire net with its opposite ends being abuttingly joined to each other so that said endless conveyor belt has substantially uniform thickness over its length and breadth.
 5. An apparatus as claimed in claim 1, further comprising second heating means, provided in said furnace and disposed downstream of said first heating means, for heating the cross-linked sheet to a temperature so that it may foam and expand. 